Effect of lithotripsy on holmium:YAG optical beam profile

Purpose: To determine the effect of holmium:YAG lithotripsy on the optical beam profile. Materials and Methods: Beam profiles of the laser light from holmium:YAG optical fiber systems were characterized with a pyroelectric camera. Beam profiles were measured with 272-mum and 365-mum optical fibers both straight and bent to simulate lower-pole ureteronephroscopy. Struvite calculi were irradiated. Beam profiles and energy outputs were characterized for the fibers before and after ablation. Ablation crater geometry was characterized with optical coherence tomography. Results: Undamaged, straight fibers produced a near-Gaussian beam profile. Craters showed a similar near-Gaussian shape. Undamaged, bent 272-mum fibers produced a near-Gaussian beam but slightly flatter profile than the straight fiber. The bent 272-mum fiber transmitted 99% to 100% of the energy, similar to the 100% transmission of the straight fibers. After ablation, measured energy output dropped by 30% within 50 pulses at 0.2 J pulse energy. The damaged fibers produced irregular beam profiles with hot spots. Craters showed irregular contours. Conclusions: During Ho:YAG lithotripsy, the beam profile at the optical fiber tip approaches a Gaussian distribution. This shape corresponds to the crater produced on the stone surface. With further ablation, the beam profile becomes erratic and unpredictable, with loss of lithotripsy efficiency. The findings provide further insight into the photothermal mechanism of Ho:YAG lithotripsy.